This invention relates in general to integrated circuits with thin film resistors, and, in particular, to a method for forming thin film resistors of different materials over a semiconductor substrate.
Thin-film resistors are generally considered to be more precise than resistors made by diffusion or by deposited polysilicon. This is due to the superior (lower) temperature coefficient of resistivity, and voltage coefficient of thin-film resistors, when compared to diffused resistors, and polysilicon resistors. Also, thin-film resistors are formed much later in the process flow (usually just prior to the interconnect metallization deposition), than diffused resistors, and polysilicon resistors. The effects of subsequent process steps on the properties of a thin-film resistor are thus minimized.
In a typical thin film process, a dielectric layer is deposited over the semiconductor substrate, thin film resistor material is uniformly deposited on the dielectric layer, and the thin film resistor material is patterned into a geometric configuration that yields the desired resistance value. Thin film resistors are trimmed precisely by a laser trimmer to within a very small deviation from the desired resistance value.
Thin film, trimmable resistors may be used in high/low power analog and low power digital integrated circuits. In the past, integrated circuits have generally restricted the devices in any given circuit to either high/low power analog or low power digital applications. However, modern circuits integrate both analog and digital applications on a single chip. See, for example, U.S. Pat. No. 5,369,309, the entire disclosure of which is here and incorporated by reference. That patent describes an A-to-D converter and describes the simultaneous fabrication of high power and low power devices in a substrate. Precision thin film resistors are used to manufacture analog to digital converters, band-gap reference circuits, and subscriber line interface circuits (SLIC) for telephone systems.
Until recently, telephone line cards have been typically housed at locations where large power generators were readily available (i.e. central offices). Thus, the amount of power required for operating the SLICs contained in the line cards was relatively unimportant.
Optical fibers have very wide bandwidth and are capable of handling a large number of signals. A large number of telephone conversations and/or blocks of data transmission can be concentrated into a much smaller number of telephone cables made with optical fibers.
Unfortunately, optical fibers are unable to carry the D.C. power required to operate the telephone sets. It is impractical to supply power at the subscriber site. Such subscriber telephones would be incompatible with existing phone sets, and would depend on the power companies. In the event of a loss of utility power, a subscriber's phone would not work. The problem is solved by providing remote sites with relatively small power capabilities at several locations within the neighborhoods to compensate for the optical fiber D.C. power deficiency. With this new arrangement, signals from the central offices are transmitted to the appropriate remote sites where the line cards (and hence the SLICs) now reside. The SLICs then provide the signals and the power to the subscriber phones (and extensions) at the subscriber sites.
Since these remote sites have limited power generating capabilities, the power consumed by the SLICs has become a critical factor. Thus, new SLIC designs are expected to provide the high power required to make telephone sets operate properly, while minimizing the power consumption necessary to operate the SLIC itself. Furthermore, the relief in performance requirements that would logically follow from a closer proximity to the subscriber, have not materialized because expanded duties have been imposed on the SLIC function.
Low power applications require a material with high sheet resistance such as silicon chromium (sichrome, or SiCr). That material has a sheet resistance on the order of 2.0 kOhm per square. For high power applications, the material of choice could be nickel chromium (nichrome, or NiCr) which has a sheet resistance of about 200 Ohms per square. There is an order of magnitude of difference between the sheet resistance of sichrome and the sheet resistance of nichrome. If one uses higher power nichrome resistors for low power applications, there would be an insufficient amount of substrate area to form the low power resistors, which typically range between 10 kOhms to 500 kOhms. This is especially critical in low-power applications and in circuits that require a critical ratio match for both low value resistor sets and high value resistor sets. In the latter case, a diffused or implanted resistor is typically used for the low value portion of the combination. A diffused resistor is made by connecting a metal interconnect (such as aluminum) to a silicon diffusion or implant through contact apertures cut through a field oxide (typically SiO2) at the ends of the diffuision or implant. The silicon diffusion or implant serves as the resistor material.
As such, there remains a long felt and unfilled need to provide resistor sets in which high value resistors are precisely matched and low value precise resistors are also precisely matched. There is also a need for sets of high and low power resistors with very low temperature coefficients as well as low voltage coefficients over the operating range of the device. There is also a need for precision resistors with significantly different sheet resistance values that are formed on the same die or wafer. There is a further need for thin film resistors of different materials formed on the same dielectric layer or formed on different, dielectric layers.